My question is how do you know when you should be adding equations together (The step referenced above). I know there has to be a fundamental concept I am forgetting. I think I tried the solving for a and then substituting the answer into the other equation, therefore treating each equation as if they are mutually exclusive. Please explain the correct approach/rationale and how to make the correct determination. Thanks.

The most obvious reason to add equations together or subtract one from another is having one of the variables with the same constant multiplier (or opposite, e.g. 3 and -3).a + 3b = 72a – 3b = 7ora + 3b = 72a + 3b = 7

If we consider this specific question then you can see that we could subtract one from another right from the beginning :a – (b/3) = 1/3 –a + (b/4) = 3/2

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